A capacitive humidity sensor having a humidity sensitive section including a pair of comb electrodes covered with a humidity sensitive layer through a protective layer, and a method of manufacturing the sensor are disclosed in, for example, U.S. Pat. No. 6,580,600 corresponding to JP-A-2002-243690.
FIGS. 5A to 5C show a capacitive humidity sensor 100 disclosed in U.S. Pat. No. 6,580,600. The sensor 100 includes a humidity sensitive section 10 and a circuit element section 20. The humidity sensitive section 10 includes a comb type capacitive element having a pair of comb electrodes 10a, 10b. The comb electrodes 10a, 10b are formed on one surface of a semiconductor substrate 1 through an insulating layer 2 to face each other with a predetermined gap, so that comb-teeth portions of the comb electrodes 10a, 10b are alternately arranged. The circuit element section 20 includes a reference capacitance section 21 and a signal processing circuit section 22. The reference capacitance section 21 includes a stacked type capacitive element having stacked electrodes and the insulating layer 2 as a dielectric.
The comb electrodes 10a, 10b are made of aluminum (Al) or aluminum alloy. The comb electrodes 10a, 10b are formed in the same process as wiring of a semiconductor element, which is formed at a different position of the substrate 1, is performed. Therefore, the comb electrodes 10a, 10b can be formed while the wiring is performed. A humidity sensitive layer 4 covers the comb electrodes 10a, 10b through a protective layer 3. The humidity sensitive layer 4 has a dielectric constant, which changes in accordance with a change in relative humidity. The protective layer 3 is made of silicon oxide (SiO2), silicon nitride (Si3N4), or a laminated member of silicon oxide and silicon nitride. The protective layer 3 protects the comb electrodes 10a, 10b from deterioration such as corrosion. As shown in FIG. 5A, the protective layer 3 of the humidity sensitive section 10 is a part of a protective layer covering all over a surface of the substrate 1. Therefore, the protective layer 3 of the humidity sensitive section 10 is formed while the protective layer of the substrate 1 is formed. The humidity sensitive layer 4 is made of polyimide resin, for example.
The humidity sensitive layer 4 has a relative permittivity of about 3 to 4, whereas water (H2O) has a relative permittivity of about 80. Therefore, when water molecule is absorbed in the humidity sensitive layer 4, the dielectric constant of the humidity sensitive layer 4 increases. Accordingly, the capacitance of the humidity sensitive section 10 increases. Thus, the dielectric constant of the humidity sensitive layer 4 changes in accordance with a change in the relative humidity in the ambient atmosphere, and accordingly the capacitance of the humidity sensitive section 10 changes.
In contrast, the capacitance of the reference capacitance section 21 remains constant, even when the relative humidity in the ambient atmosphere changes. That is because the reference capacitance section 21 is not covered with the humidity sensitive layer 4.
The comb type capacitive element of the humidity sensitive section 10 is connected in series with the stacked type capacitive element of the reference capacitance section 21. Therefore, the capacitance change of the humidity sensitive section 10 can be detected as a variation of a ratio between voltages applied to the respective capacitive elements. Then, the relative humidity is calculated in the signal processing circuit section 22 based on the detected variation of the ratio between the voltages. Thus, the sensor 100 measures the relative humidity in the atmosphere. The signal processing circuit section 22 is integrally provided in the sensor 100 so that the sensor 100 has a small size and is manufactured at low cost.
However, as disclosed in US 2003/0179805A1 corresponding to JP-A-2003-270189, it has been showed that the sensor 100 exhibited hysteresis in a variation of capacitance. The hysteresis of the sensor 100 means a difference of the amount of change in capacitance of the humidity sensitive section 10 between during an increase in relative humidity and during a decrease in relative humidity. It has been considered that the hysteresis was caused due to grooves 3m in the protective layer 3. Specifically, it is difficult for the water absorbed in the humidity sensitive layer 4 located inside the grooves 3m to move (evaporate) when the relative humidity decreases, because the grooves 3m become narrower toward the top. As a result, the change in the capacitance of the humidity sensitive section 10 is delayed than the change in the relative humidity.
In a capacitive humidity sensor disclosed in US 2003/0179805A1 corresponding to JP-A-2003-270189, a separation distance between adjacent comb teeth portions of a pair of comb electrodes is widened in order to prevent the hysteresis. In this case, water absorbed in a humidity sensitive layer can easily move and the hysteresis can be reduced. However, when the separation distance between the adjacent comb teeth portions is widened, electrode density per unit area decreases. Therefore, a capacitance between the comb electrodes decreases, and sensitivity of the humidity sensitive section decreases.